This invention relates to the control of metallic corrosion by the well known power-impressed current cathodic protection method. More particularly it relates to the anodes used in this method or system of metal protection.
Cathodic protection of metal structures is an old art. In 1824 A.D. Sir Humphrey Davy described how zinc anodes could be used to prevent the corrosion of copper sheathing on the wooden hulls of British naval ships. The method ultimately failed and interest waned for almost 100 years before it was first used successfully to protect underground pipes. (See Corrosion, vol. 2, p. 11:1, edited by L. L. Shreir, published by Newnnes-Butterworth of London and Boston in 1976.)
Cathodic protection of metal structures from corrosion is applied by one of two methods, that is by the electrical power impressed current method or by the sacrificial anode method. In either method a second metal which serves as an anode is maintained in contact with the same environment as that contacting the metal structure to be protected from corrosion and both the metal to be protected and the anode must be electrically connected. Since the environment must conduct ions for corrosion to take place, it can range from highly conductive substances, such as sea water, to less conductive environments such as earth and concrete.
Corrosion of the metal in such environments is a well-known phenomenon of nature which occurs as the metal gives up electrons to its surrounding environment by electrochemical reactions with the environment. The concept of preventing this corrosion is to force electrons into the metal from an external power source at a rate which is at least just as great as the rate of electrons leaving the metal to become involved in the corrosion causing electrochemical reaction occurring between the metal and its environment.
Briefly, in the impressed-current system of cathodic protection, an electromotive force (EMF) is relied upon to supply electrons to the environment and thus eliminate the metal to be protected as a source of the electrons required to balance the natural requirement for electrons to the environment due to the presence of the metal therein. Both the anode and cathode are in electric contact with each other and each is in electrical contact with the electrolyte, the corroding medium.
Since a source of electrons is supplied by the power source and the source is sufficiently large to supply the needs to prevent electron flow from the metal to be protected, it makes no difference whether the anode is naturally more electropositive than the metal to be protected. However, as with sacrificial anodes, even the anodes of an impressed-current cathodic protection system will corrode away rapidly unless special anode material is selected. Since in many applications where cathodic protection is employed, it is difficult and expensive to replace the cathodic protection anodes frequently, it is desirable they be prepared from a material which has resistance to the corrosive environment. The commonly used cathodic protection anode materials and their consumption rates are shown in Table I set forth immediately below.
TABLE I ______________________________________ Appropriate Consumption Rate Anode Material* (kilogram/amp-year) ______________________________________ coke breeze 0.5 zinc 10.8 aluminum 4.5 graphite 0.1-1.0 Lead/platinum 0.09 Pb--6Sb--1Ag 0.09 Cast Iron 4.5-6.8 Iron 9.5 Steel 6.8-9.1 High-silicon iron 0.25-1.0 High-silicon/chromium iron 0.25-1.0 DSA-Titanium** 4.75 .times. 10.sup.-6 Platinum 8.63 .times. 10.sup.-6 Platinized titanium 8.76 .times. 10.sup.-6 Platinized niobium Ditto approx. Platinized tantalum Ditto approx. ______________________________________ *The above information was gathered from pages 11:34-11:56 of vol. 2 of Corrosion cited above. **DSA is the shortened form of the proprietary name, Dimensionally Stable Anode. It is owned by the Diamond Shamrock Technologies Company. The material for this anode was originally developed by Henri Beer, of Antwerp, Belgium. See U.S. Pat. No. 3,632,498. The anodes claimed therein include coatings comprised of a mixture of an oxide of a film forming metal and an oxide of a platinum group metal on a base chosen from a select group of metal bases. In the trade the DSA anode has come to be associated with the specific anode having a titanium substrate with a coating of the oxides of ruthenium and titanium, with the weight proportion of these oxides being about equal.
As can be seen from the table above there is about a million-fold decrease in anode consumption rate (conversely a similar increase in expected life of the anode under the conditions) when moving from the cheaper metals to the expensive platinum and platinum group metal coated anodes, the DSA anode being an anode containing ruthenium in its surface coat. However, these anodes are tremendously greater in cost per unit weight than the cost of the other types of anodes.
Besides the consumption rate and cost involved in the selection of these anodes, there are other criteria to be considered. For example, power efficiency is a criterion to be considered; that is, how many amperes of current per volt per unit of surface area an anode will provide. At the same voltage, it is known that different anode materials produce different electrical current densities. It is therefore desirable that greater power efficiency be achieved, that is, a high ratio of amperes per unit area per volt applied.
An additional problem associated with the platinum and with the platinum coated anodes is that the platinum will complex with some organic compounds and be very rapidly consumed. See page 6:11 of Corrosion cited above. Of course, this complexing destroys the long-term capability of the anode to control the corrosion of the metal structures it is intended to protect. Thus very expensive organic chemical processing equipment can rapidly corrode in industry with great loss of capital and sales.
Thus it would be advantageous to have anodes for use in impressed cathodic protection systems which have a low consumption rate, which are economical, which have a high power efficiency, and which are not known to complex with organic compounds. The present invention provides such anodes.